Each embodiment relates to the transformer and the like of a gradient field power supply used for a magnetic resonance imaging apparatus. A magnetic resonance imaging apparatus is an apparatus which visualizes the chemical and physical microscopic information of a substance or observes a chemical shift spectrum by using a phenomenon in which when a group of nuclei having a unique magnetic moment is placed in a uniform static field, they resonantly absorb the energy of a radio-frequency magnetic field that rotates at a specific frequency. Such a magnetic resonance imaging apparatus is very effective as a method of noninvasively obtaining an anatomical cross-sectional view of a human body. This apparatus is widely used as a diagnosis apparatus for a central nervous system such as the brain surrounded by the skull.
Such a magnetic resonance imaging apparatus includes gradient field coils, and a gradient field power supply apparatus for supplying currents to the gradient field coils as components of a mechanism for forming magnetic fields (gradient fields) whose strengths linearly change in the directions of spatial orthogonal axes (i.e., the x-, y-, z-axes).
FIG. 10 is a view showing the arrangement of a general gradient field power supply apparatus. As shown in FIG. 10, a gradient field power supply apparatus 50 includes an X-channel (Xch) gradient field power supply 51, a Y-channel (Ych) gradient field power supply 52, and a Z-channel (Zch) gradient field power supply 53. As the driving power supply of the gradient field power supply apparatus, for example, three-phase AC 400-V power is used. The power supply transformers in the gradient field power supply apparatus 50 transform and distribute the three-phase 400-V power supplied to the gradient field power supply apparatus 50 to supply the power to each of the gradient field power supplies 51, 52, and 53. The gradient field power supplies 51, 52, and 53 independently operate under the control of a control apparatus to supply currents to the gradient field coils for generating gradient fields in the corresponding coordinate axis directions.
The conventional gradient field power supplies, however, have the following problems. That is, for example, as shown in FIG. 11, the power supply transformers of the gradient field power supply apparatus of the conventional magnetic resonance imaging apparatus have single-phase secondary windings wound around each of three-phase (U-phase, V-phase, and W-phase) primary windings. For example, as shown in FIG. 11, the single-phase secondary windings, i.e., the secondary windings corresponding to the respective channels, are not spatially uniformly arranged between the U-phase, V-phase, and W-phase (considering, for example, the X channel (Xch), the number of X-channel (Xch) secondary windings wound around the W-phase primary winding is smaller than that of X-channel (Xch) secondary windings wound around each of the U-phase and V-phase primary windings). Therefore, offsets occur in loads on the respective phases of the primary windings corresponding to the respective output channels, i.e., the X channel (Xch), Y channel (Ych), and Z channel (Zch). For this reason, offsets occur in a load on each phase of the primary winding corresponding to a specific output channel, resulting in an increase in voltage variations at the time of output.
In addition, with a recent tendency toward an increase in output current, the voltage of the gradient field power supply apparatus tends to frequently vary. For this reason, the magnetic field imaging apparatus has the function of protecting a subsequent stage by interlocking in accordance with the value of an output voltage. If, however, the width of voltage variation is large, it is impossible to execute any imaging method in which a voltage value exceeds a threshold for interlocking. When an imaging method like echo planar imaging (EPI) is executed, since a current concentratedly flows in the read channel, offsets tend to occur in the respective phases of the primary windings corresponding to a specific output channel.
There are provided a gradient field power supply apparatus which can supply power more stably than the prior art, even with the occurrence of offsets in output currents between output channels, by leveling power which the respective phases of the primary windings of the power supply transformer secure for the respective output channels, and a magnetic resonance imaging apparatus including the gradient field power supply apparatus.